我有以下两张表:
1. Lecturers (LectID, Fname, Lname, degree).
2. Lecturers_Specialization (LectID, Expertise).
我想找最专业的讲师.
SELECT
L.LectID,
Fname,
Lname
FROM Lecturers L,
Lecturers_Specialization S
WHERE L.LectID = S.LectID
AND COUNT(S.Expertise) >= ALL (SELECT
COUNT(Expertise)
FROM Lecturers_Specialization
GROUP BY LectID);
但当我try 这个时,它是有效的:
SELECT
L.LectID,
Fname,
Lname
FROM Lecturers L,
Lecturers_Specialization S
WHERE L.LectID = S.LectID
GROUP BY L.LectID,
Fname,
Lname
HAVING COUNT(S.Expertise) >= ALL (SELECT
COUNT(Expertise)
FROM Lecturers_Specialization
GROUP BY LectID);
原因是什么?谢谢